Unable to stack

[descott12]

Hello,
I had a pretty unproductive night trying to stack with my ASI178MM on my Evolution 8 at f10 and also at f6.3. I have attached some screenshots and I am hoping there is something obvious that I am doing wrong. This has always worked quite well in the past but I kept getting "not enough star" errors in the log.

A few things I noticed are the the FWHM values were either very high or ??? and it would fail to stack when the FHWM would go to ???. The screen clearly shows many stars and the focus was good (I had a really good bahtinov appearance).
I also tried my 294 color and it seemed to do better although there were a few FWHM ??? at times and I normally don't see that. So what causes a high or ??? value for FWHM and I had the FWHM filter turned off to try to fix this but that didn't help.

I tried messing around with the stacking parameters but they didn't seem to help. The only thing that had any positive affect was going to BIN 2 or 3.

Any ideas? Thanks in advance

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[descott12]

One more screenshot

[admin]

Hi,

The last screenshot makes the cause very clear – no stars being detected which prevents SharpCap from being able to align each frame with the previous ones. By the look of things, the most likely cause of this is that the focus is fairly soft leading to the brighter stars being very large blobs (too large to be detected properly).

Some things to do to make sure it works better next time –

Turn off the display histogram stretch on the right-hand side – this is brightening the stars and making them look clear to you in the view when they may not be particularly bright and maybe hard to detect.

Do your best to improve the focus and get more points like stars

Increase the maximum star width parameter on the alignment tab and also tick the checkbox on that tab that will highlight the stars that being detected so you can see how the changes you make to the star detection parameters affect the results of the star detection.



Hope this helps, Robin

[descott12]

Hi Robin,
Thanks for the quick response. Interesting about the focus because I was using a b-mask but I did switch back and forth with the focal reducer so I probably didn't refocus well between switches.
Thanks

[mAnKiNd]

On top of what Robin said, whenever I have this issue, I find I need to revisit the black point threshold value inside FWHM measurement to figure out what that value should be with a star. If it's too low or high, I don't get the right FWHM reading and sometimes it doesn't even detect any stars. Once I figure it out, I set that value in the live stack settings and it starts finding stars to align. However, like Robin said, you need good focus to start with.

You can consult the excellent manual at:
https://docs.sharpcap.co.uk/3.2/#Focusing%20Procedure
Under "Setting the correct black level"

Hope this helps,
Minos

[descott12]

Thanks for the link. There was quite a bit I was not aware of...I should do much better next time

[chongo228]

I agree with the stars looking bloated and the black point needing to come up as stated above.

Your gain setting of 415 seems very high. This would cause noise and the stars to swell up even if you're in perfect focus.

I'd go with longer time and less gain. I usually stick with 10 to 45 seconds with 139 gain for LRGB and 1-2 min with a gain of 200 for narrowband. I have a ASI1600 so i'm not sure how the settings would translate to your camera.

[descott12]

Hello All,
I had another go at this using alot of your suggestions. I have verified that my focus is pretty good using both the bahtinov and the FWHM tools, I played with the max star width (min 2 to max 32) and the black point quite a bit. I was able to make some progress but nothing, other than going to bin 2 from bin 1, had any consistently positive results.

Thanks in advance if somebody can explain some of my questions below. This is pretty interesting stuff but I feel like I am not quite understanding where it/I am failing.

The effects of setting the black point seem almost random. Is there a rule of thumb for setting that? Sometimes 15 was best, sometimes the default was best and sometimes much higher. No pattern that I could see.

In all cases where alignment and stacking failed, the FWHM screen would show all ???. Sometimes it would be fine and then go all ??? without any changes. See screenshot. Is this simply due to unpredictable seeing?. This seems to be the root of my problem.

Is there a reason why I never have issues at f2 with my hyperstar but only at slower speeds?
Does binning help counteract the slowness of f10 or 6.3 compared to f2

On the screenshot, the red 4.98 FWHM doesn't make sense. It has a better/lower score but it turned red and it was not aligned/not enough stars. Why is that?

Finally, I have attached the focusing screens. The b-mask was jumping around quite a bit but it was settling out close to 0.0 Is this level of focus adequate?

Thank you

[admin]

Hi,

The '???' values for the FWHM occur when there are not enough stars detected to calculate a good average. On the other hand, the read appears in that graph for frames that have not been added to the stack for one reason or another – that could include failure to align or the FWHM being too high. From the look of your screenshots you have the FWHM filter turned off (the box is not checked), so it's most likely that the problem was a failure to align.

The best setting for the black level parameter can be found by looking at your histogram for the individual frames – look at the percentage fraction of maximum histogram value that the biggest peak appears at (the peak that represents the dark background of the frame) and set the black level parameter for the live stacking to be roughly the percentage position of the right hand side of that peak.

I think it's hard to tell whether the bahtinov mask focusing was accurate from the graph – that's usually better done by looking at a screenshot of the bahtinov pattern.

Certainly, even in your last screenshot below that shows some success in stacking it looks like you only have about 10 stars being detected – if you have a frame that is disturbed by poorer than the average seeing then you could easily imagine two or three stars being detected in that frame which would not be enough to align it correctly.

It may well be worth giving binning a try – if you are using a 178 camera with an SCT at F/10 then stars will naturally be rather spread out due to the small pixels of the camera and the long focal length.

Cheers, Robin

[descott12]

Thanks as always Robin,
I think this is boiling down to a long focal length and not enough stars in the field.